There has been developed an artificial photosynthesis technology that replicates photosynthesis of plants and electrochemically reduces carbon dioxide by artificially using renewable energy such as sunlight to produce a storable chemical energy source from viewpoints of energy problems and environmental problems. An electrochemical reaction device that enables the artificial photosynthesis technology includes, for example, a photoelectric conversion layer using a semiconductor, an oxidation electrode that oxidizes water (H2O) to generate oxygen (O2), and a reduction electrode that reduces carbon dioxide (CO2) to generate carbon compounds. In such an electrochemical reaction device, the oxidation electrode and the reduction electrode which are electrically connected to the photoelectric conversion layer are immersed in a solution (electrolytic solution) containing water in which CO2 is dissolved, to cause a reduction reaction of CO2.
The oxidation electrode has, for example, a structure in which an oxidation catalyst which oxidizes H2O is provided on a surface of a metal base material. The reduction electrode has, for example, a structure in which a reduction catalyst which reduces CO2 is provided on a surface of a carbon base material. The oxidation electrode and the reduction electrode are electrically connected to, for example, a renewable energy power supply such as a solar cell. The reduction electrode obtains a CO2 reduction potential from the solar cell, thereby reducing CO2 to generate carbon compounds such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), methane (CH4), ethanol (C2H5OH), ethane (C2H6), and ethylene glycol (C2H6O2).
There is a problem that a production amount of a reduction product of CO2 is easy to be continuously lowered when the above-stated reaction where CO2 is electrochemically reduced is carried out for a long time. As a method to solve the problem, it is proposed to recover the production amount of the reduction product of CO2 by electrochemically processing the reduction electrode. For example, when the reduction reaction of CO2 is electrochemically driven, it has been reported that the production amount of the reduction product of CO2 is maintained by applying voltage or potential in a step state. However, the reduction electrode is damaged by repeating the reactivation process as stated above. Accordingly, it is impossible to stably operate the electrochemical reaction device for a long time such as, for example, for 100 hours or more according to the conventional reactivation process.